ACADEMIC OPTICAL INTERCONNECTS GROUPS

In recent work, several academic groups, including ours, have demonstrated optical-interconnect systems in the lab [12-26].   Due to the challenges and cost of implementing large scale fiber systems, these have tended to be free-space interconnects. The primary differences between the various projects have tended to be in two areas: the choice of optoelectronic devices – lasers or modulators, p-i-n or metal-semiconductor- metal (MSM) detectors – and the design of the optical system itself.   Since this thesis deals with the question of optoelectronic modulators, a discussion of the advantages and disadvantages of a VCSEL-based approach will appear in Chapter 3.   Let us briefly

describe some recent representative examples of the academic groups’ work on optical- interconnect systems.   (A more exhaustive review is given in a few recent published special issue journals [27, 28].)

The Esener Group at University of California at San Diego has designed and demonstrated an optical interconnect system with silicon chips mounted on a PCB as usual [14].  On top of the chips, a 4-f imaging system was implemented in commercially- available  bulk  macro-optics.    As  a  test  system,  this  project  was  able  to  show  the feasibility of combining multiple types of materials, such as Si, GaAs VCSELs and MSMs,  ceramics,  glass  lenses  and mirrors,  PCBs,  etc.,  and  of  achieving  a  working system.  Future systems would be expected to use microlenses instead, in order to reduce cost  and improve  scalability.    The advantage  of this  scheme  is  its  use  of currently available products in a relatively compact design.  Drawbacks are that it is still a bit too bulky for practical use and the published speed of 250 MHz is far too low.  Of course, this would improve as CMOS technology improves and the optical system is not sensitive to the bit rate.  The scalability of a system with bulk optics is also an unresolved question.

The work of Jurgen Jahns and Matthias Gruber in planar optics has the potential to solve this problem of the scalability of optics [24, 25, 29, 30].  Using a glass substrate, diffractive optical elements (DOEs) can be etched into the surface.  CMOS chips can then be flip-chip bonded with high accuracy onto this glass substrate.  Routing of electrical signals and power lines could be achieved by running wires along the glass surface, as well.  Diffractive optical systems are able to perform more complicated routing functions, instead of relying on simple bulk optics to perform the same function to the entire array of signals.  This technology can also be integrated with standard PCBs and fiber-based optics as shown below.

Fig. 2.1. Planar optics of Jahns and Gruber (image courtesy M. Gruber)

The group of Hugo Thienpont has also been investigating microoptical systems [13, 17].   Their work in materials such as polymethylmethacrylate (PMMA) using the technique of deep proton lithography has yielded high quality results, especially for intrachip or short distance intra-MCM optical interconnects.   Such short distance interconnects may be useful for signaling as well as clock distribution.

Finally, the Miller group has utilized bulk optics to design a free-space optical interconnects test system [20, 22, 31].   The purpose was not to study packaging technology, but instead to characterize important system parameters in optical interconnects and to demonstrate the benefits of using certain schemes.  Using a short- pulse  modelocked  laser  as  the  light  source,  we  demonstrated  improved  receiver sensitivity [20, 32, 33], optical link latency reduction [31, 32], and WDM optical interconnects using spectral slicing [22].   The issue of clock distribution was also addressed using several schemes, including the so-called “receiver-less” design [20, 33,

34].   Many of these results would apply equally well to the integrated planar optical systems being studied by Jahns and Gruber, for example.